The present disclosure pertains generally to image display systems using cascaded or multistage display devices and more particularly to display systems that use a cascade of digital display devices.
A cascaded or multistage display system is an optical system where the light output of one display device becomes the input to another display device. The output of a cascaded display is approximately equal to the pixel-by-pixel multiplicative product of the cascaded display devices. Generally, cascading two devices is sufficient, but more could be cascaded for further improvement. Also, in multi-color systems, there may be multiple sets of cascaded displays, as may be required to separately modulate one or more colors per cascade.
Using display devices in a cascade provides advantages, such as:                1. Increased contrast ratio (i.e. dynamic range). The contrast ratio of the cascade is the product of the contrast ratios of the individual displays. Thus, two devices that individually produce a 500:1 contrast ratio, in cascade can produce a 250,000:1 contrast ratio.        2. Increased bit depth. The cascaded bit depth is approximately the sum of the bit depths of the individual displays.        3. Reduced pulsed width modulation (PWM) artifacts (in PWM-based displays).        4. Reduced dither noise (in displays that use dither).        5. Reduced performance requirements for one or both display devices, as the work is shared between the multiple devices. Thus, two low capability devices in cascade can create a better image than either device alone.        
Display devices convert electrical signals into light levels that make up the displayed image. Digital display devices are a subset of display devices, and are capable of displaying a finite number of discrete light levels, or gray shades, per pixel. Binary (two state) displays are a subset of digital display devices that can display only one of two light levels per pixel at any instance in time, the two light levels being fully ON (white) or fully OFF (black).
Examples of digital display devices include: the Digital Micromirror Device (DMD) from Texas Instruments (Dallas, Tex.), the Digital Liquid Crystal on Silicon (D-LCOS) device, the VueG8 technology from Syndiant (Dallas, Tex.), and the Plasma Display Panel (PDP), and light emitting diode (LED) displays. Some analog imaging devices can also be operated as a digital display, including the D-ILA device from JVC-Kenwood (Kanagawa, Japan).
An image is composed of rows and columns of pixels. Each pixel of a frame has associate data that represents the light intensity and, in multicolor displays, the color of the pixel. The data is comprised of one or more binary bits (zeros or ones). The value each bit represents may be a binary weighting (powers of 2), or some other, possibly arbitrary, weighting.
In order to enable a digital display system to show more gray shades than the intrinsic capabilities of the digital imaging device, some sort of modulation in time of the digital imaging device is required, e.g. PWM. The digital imaging device is modulated with a signal such that the intensities of the displayed pixels average to the desired gray shade, over a time frame short enough that the human vision system will perceive these average pixel levels, rather than the modulating signal.
One approach to generating this digital imaging device modulating signal is to convert the incoming image data into bit planes, with each bit plane representing a bit weight of the intensity values. If each pixel's intensity is represented by an N-bit value, each image frame will have N bit planes. Each bit plane has a 0 or 1 value for each pixel. The bit weight is often binary (i.e. a power of two), but is not limited to binary ratios. For example, a 4-bit video signal may have 4 bit planes, with bit weights of 0.5, 0.25, 0.125, and 0.0625. Equivalently, the weights may be stated in integer form: (8, 4, 2, and 1), as the salient aspect of the bit weights is their ratios.
Using multi-level halftoning (multitoning), the incoming image data can be converted to a representation using more, or fewer, bits per pixel. Multitoning can also convert from a binary (bits are powers of 2) representation to a representation with arbitrary weights per bit. This provides the ability to use arbitrary numbers of bit planes, with arbitrary bit weights, as will be apparent to one skilled in the art of multitoning.
Each image is displayed for an amount of time called the frame time. An image frame can be subdivided into time slots, known as bit segments. Each bit segment is displayed for an amount of time that is proportional to the desired bit weight of the bit segment. The bit segments can be all the same weight, or they can vary by segment. If the illumination is variable, this will also affect the bit weight of the bit segments. Some digital displays (e.g. DMD) can produce shorter bit segments if one or more adjacent bit segment is lengthened. Short bit segments are desired for high effective bit depth, but require more data bandwidth and device speed.
Each bit plane is displayed in one or more bit segments, with the bit weight of each bit plane being equal to the sum of the bit weights of the associated bit segments. The length of time each bit plane is displayed is proportional to the bit weight of the bit plane. During a bit segment, all the pixels of a binary display will be ON or OFF, depending on the related bit plane data.
Due to display device characteristics, there may be a time skew across the device, resulting in the bit plane data being displayed at different times in different areas. Display devices may update the bit plane data pixel-by-pixel, line-by-line, or in blocks, depending on the device capabilities.
Multi-bit bit planes can be used to operate digital displays that can produce more than two shades (ON and OFF). The number of bits per bit plane is a function of the number of possible shades provided by the digital display device, as will be apparent to one skilled in the art.
The arrangement of the bit segments in time and their associated bit weights and bit planes, is called the bit sequence (the “sequence”). The design of bit sequences involves reconciling the various aspects of display quality, including bit depth, dark noise, bandwidth, light efficiency, color artifacts, and motion artifacts.
Typically, not all possible combinations of bit planes are used. For example, a cinema display running at 24 frames per second and using a DMD with an average bit segment of 170 us can display about 260 bit segments per frame. Using one bit plane per bit segment, if every possible combination of bit planes was used, there would be 2^260 combinations, or about 10^78. This is obviously more than is required or practical. In addition, many combinations are redundant, as they have the same or very similar bit weight. In practice, a subset of combinations is chosen, with a total count ranging from dozens to hundreds of combinations. Each chosen combination of bit planes, termed a “bit code”, has an aggregate bit weight, and thus a gray level, as well as a bit vector representing the bit planes that should be ON, or ‘1’.
Almost all sequences used in applications at or below 120 frames per second, use repeated sub-sequences of identical, or nearly identical, arrangements of bit segments and bit planes. This provides an opportunity to apply multiple halftone or multitone images to a secondary display. For example, DMD cinema displays operating at 24 frames per second are actually displaying each image four times, for a 96 Hz. sub-sequence rate. This gives the opportunity to have four halftone images, which, when integrated by the human visual system, gives the appearance of two extra bits of bit depth. Another way of describing this effect is that the halftone dots appear at 25% contrast, rather than 100% contrast. (See U.S. Pat. No. 6,774,916 and U.S. Pat. No. 7,446,785 and U.S. Pat. No. 7,576,759, incorporated herein by reference).
In the context of a cascade display system, a primary display is a display that is in sharp focus. A secondary display is a display that is not in sharp focus. A cascade display system consists of an illumination source, one or more primary display devices, zero or more secondary devices, one or more relay optics, and supporting optics, mechanics, and electronics.
Cascade displays using digital display devices cannot use conventionally designed bit sequences without unwanted interactions between the display devices. If the display devices are not frame locked, strobing (i.e. low to medium frequency intensity changes) and non-deterministic behavior (i.e. the same image data gives different results on successive viewings) can occur.
Even if frame locked, conventional sequences will interact in a cascade display, causing undesirable artifacts including nonlinearity (i.e. discontinuities in images that should be smooth), flicker (undesirable intensity changes at a sub-multiple of the frame rate), banding (i.e. on devices that have regions with different timing, such as phased reset DMDs, the regions can have differing behavior), severe color errors, and dither visibility, particularly when 1-bit dither patterns are used.
One approach to avoiding artifacts is to restrict the secondary display to use a 1-bit sequence with halftoning. The disadvantage of this is the visibility of the halftone dots, which are either full ON or full OFF.